It is known in the art that a ship's hull friction resistances in water will decrease by supplying air bubbles to the surface of a moving hull a1.
There are two types of means to send air to the bubble generator prepared in the ship hull's bottom (lateral surface); one type is a closed type which depresses the air-liquid interfaces in an air charging line to an air bubbles genesis region by means of a compressor; and a second type is an open type system that depresses the air-liquid interface in an air charging line to an air bubbles generator region by negative pressure.
Prior art document 1 (JP 2008-120246 A) and prior art document 2 (JP 2008-143345 A) describe a closed type system. Prior art 1 describes a structure which attaches a flat board so that it covers an air hole, and Prior art 2 describes the structure which introduces air into the water in a uniform state, attaching a baffle plate which covers gas pressure jets.
Here, the flat board described in Prior Art 1 and the baffle plate described in Prior Art 2 do not generate negative pressure as described in the documents concerned.
The closed type forcedly spouts air and as a result the air bubbles will inevitably become large. In order to effectively reduce frictional resistance, air bubbles must stay on the surface of hull for a long time. For this reason it is required that the bubble diameter should be as small as possible. Prior Art 3 (JP 2002-2582 A) describes that such microbubbles are generated according to the Kelvin-Helmholtz-Instability phenomenon.
Namely, in Prior art 3 in the panels of the ship's bottom (submerged surface) a recess is installed. This recess is connected together with a gas inlet tube to supply air to the recess and has a wedge-shaped negative pressure formation part attached to the upper stream side of the recess, which causes the Kelvin-Helmholtz-Instability phenomenon to make microbubble in the recess. Moreover, in Prior art 4 (JP 4070385), the technology of employing a wing is disclosed as a means which makes microbubbles instead of the wedge-shaped negative pressure formation part of Prior art 3.
[Prior art 1] Laid Open Patent JP2008-120246A
[Prior art 2] Laid Open Patent JP2008-143345A
[Prior art 3] Laid Open Patent JP2002-2582A
[Prior art 4] U.S. Pat. No. 4,070,385